Importance of Human Norovirus

Human noroviruses are a highly infectious group of viral pathogens that cause epidemics of acute gastroenteritis annually in the United States and worldwide. Illness is characterized by a rapid onset of vomiting and/or diarrhea, nausea, and abdominal pain that is short-lived, generally lasting from 24 to 72 hours. However, noroviruses may be shed prior to the onset of symptoms and in feces for several weeks after the illness has subsided (12).

The Use of Surrogate Viruses for Human Norovirus Studies and Efficacy Testing

Despite their prevalence in the human population, noroviruses have proven difficult to successfully propagate in a cell culture system. Therefore, MS-2 bacteriophage, feline calicivirus, and more recently murine norovirus have been employed as models for human noroviruses in virucidal efficacy studies.

MS-2 coliphage is a virus that infects “male” Escherichia coli cells exhibiting the F pilus on their surface. MS-2 and human noroviruses are structurally similar non-enveloped viruses; each are comprised of icosahedral nucleocapsids exhibiting the same triangulation number (T=3). MS-2 coliphages are also exceptionally hardy and environmentally resistant. They are relatively simple to cultivate and purify, with high viral titers readily achieved in the laboratory setting under suitable conditions. Virucidal efficacy evaluations that utilize MS-2 coliphage as a surrogate for human norovirus or other non-enveloped viruses also yield rapid test results (18 to upwards of 48 hours after assay initiation relative to 5 to 7 days for the feline calicivirus surrogate). Viral plaques are visualized on an E. coli host lawn, and assay results are generally presented in units of plaque-forming units (PFU) per unit volume or per carrier. Biocidal efficacy evaluations using MS-2 as a non-enveloped mammalian virus surrogate have been published in the literature including suspension time-kill tests and surface disinfectant testing (10, 14).

Feline calicivirus (FCV) and human noroviruses belong to the same viral family, Caliciviridae, although to different genera (Vesivirus and Norovirus, respectively). FCV and human noroviruses are therefore highly comparable in terms of size and structure. The higher degree of phylogenetic relativity in conjunction with an established and straightforward cell culture assay system led to the establishment of feline calicivirus as the U.S.-EPA recommended human norovirus surrogate for virucidal efficacy testing in 2005. The host cell line for feline calicivirus is Crandell-Rees Feline Kidney (CRFK), which is readily established and maintained by way of standard sub-culturing techniques. Although the aforementioned assay period of 5 to 7 days is longer than that of MS-2, the data generated from the U.S. EPA-required TCID50 (Tissue Culture Infectivity Dose at the 50% Endpoint) assay is consistent given the proper performance of required study controls (examples: Virus Control and Cytotoxicity). Viral and toxicity titers are then assessed using an established statistical method (examples: Reed-Muench or Spearman-Karber). Feline caliciviruses have been employed in many studies evaluating each of the disinfectant classes, including alcohols, halogens, quaternary-ammonium compounds, and glutaraldehydes (9, 13, 16).

Similar to feline calicivirus, murine norovirus (MNV) belongs to the same viral family as the human noroviruses. Unlike feline calicivirus, however, MNV is grouped within the genus Norovirus, thus sharing even greater phylogenetic relatedness to the human noroviruses. In addition, MNV and human noroviruses are demonstrated enteric pathogens of mice and humans, respectively, whereas feline calicivirus infects via nasal, oral, and conjunctival routes (17). The cell culture assay for murine norovirus (MNV-1) utilizes a mouse macrophage host cell line performed either using a plaque assay or TCID50 assay (21). MNV has become increasingly used as a surrogate for human norovirus in virucidal efficacy evaluations as well.

Direct assessment of anti-viral efficacy against human noroviruses would be most ideal. However, a straightforward cell culture system allowing for detection of infectious noroviruses remains elusive. While each of the viral surrogates for human norovirus has associated pros and cons, they continue to serve as viable research models for efficacy testing. The U.S. EPA has also maintained the use of feline calicivirus as an acceptable model for human norovirus disinfectant label claims.

The Importance of Disinfection: Survival of Human Norovirus Surrogates on Surfaces and Transmission Potential via Fomites

The transmission route of human noroviruses is generally described as fecal-oral. During the short-lived bout of gastroenteritis, an infected person may shed upwards of 11-log10 viruses per gram of feces (5), usually by loose bowels or outright diarrhea. High numbers of viruses are also expelled from the acute projectile vomiting episodes commonly associated with human norovirus infections (1). During the course of these events, noroviruses are deposited onto hands either by the infected person or a caretaker; these viruses are then transferred to a variety of fomites of differing compositions and textures (2, 4, 5). Viral particles aerosolized during vomiting incidents also settle onto fomite surfaces located in the general vicinity of the event (1). The low minimal infectious dose of human norovirus [10 to 100, (1)], coupled with the demonstrated transfer of virus from contaminated surfaces to previously uncontaminated hands, indicates a potential indirect route of transmission that continues to deserve consideration from the public health and research communities.

Upon expulsion from their human host via vomit or diarrhea, noroviruses inevitably lose infectivity and viability over time due to a host of environmental parameters including temperature and humidity. The rate at which viral inactivation occurs has been studied widely for feline calicivirus (human norovirus surrogate) under varying temperatures, humidity, and surface (fomite) types. Following the trend typically seen in laboratory studies, feline calicivirus (non-enveloped) remains viable on surfaces for longer time periods than enveloped viruses (5). In comparison to true enteric pathogens such as hepatitis A virus and murine norovirus; however, feline calicivirus (a respiratory virus) does exhibit a higher rate of inactivation, particularly at extreme pH values (7). In terms of environmental stability, feline calicivirus and murine norovirus displayed similarly low inactivation patterns at 4 °C when dried onto stainless steel surfaces and in suspension (wet conditions); at room temperature; however, murine noroviruses maintained greater viability relative to feline calicivirus under wet and dry conditions (7).

An abundance of disinfection studies have been performed using the aforementioned human norovirus surrogates, with feline caliciviruses comprising a majority of such investigations. Until a viable cell culture system is developed for the human noroviruses that will allow for direct virucidal efficacy evaluations, the surrogate viruses will continue to be employed in their stead.

Whitehead, K. and K.A. McCue. 2010. Virucidal efficacy of disinfectant actives against feline calicivirus, a surrogate for norovirus, in a short contact time. American Journal of Infection Control. 38(1): 26-30.

References

Barker, J. D. Stevens, and S.F. Bloomfield. 2001. Spread and prevention of some common viral infections in community facilities and domestic homes. Journal of Applied Microbiology. 91: 7-21.

Bright, K.R., S.A. Boone, and C.P. Gerba. 2010. Occurrence of bacteria and viruses on elementary classroom surfaces and the potential role of classroom hygiene in the spread of infectious diseases. The Journal of School Nursing. 26(1): 33-41.

Cannon, J.L. et al. 2006. Surrogate for the study of norovirus stability and inactivation in the environment: a comparison of murine norovirus and feline calicivirus. Journal of Food Protection. 69(11): 2761-2765.

Costantini, V. et al. 2006. Human and animal enteric caliciviruses in oysters from different coastal regions of the United States. Applied and Environmental Microbiology. 72(3): 1800-1809.